The invention relates to a viscous coupling having a first coupling part designed as a housing, a second coupling part designed as a hub, at lest two sets of coupling plates, with the first set being non-rotatingly connected to the coupling housing and the second set(s) being non-rotatingly connected to the coupling hub, with the two sets being arranged so as to alternate axially, with at least one set of coupling plates being axially movable to a limited extent, and the remaining interior space being at least partially filled with a viscous fluid whose pressure may be predetemined by a controllable pressure generator consisting of a piston/cylinder unit.
A viscous coupling having a pump for conveying the viscous medium is known from GB 22 02 602 A. The design as described in the patent specification includes a piston pump which permits the internal pressure in the viscous coupling to be changed, with the maximum pressure and the predeterminable pressure being set by two control valves. One of the disadvantages of this design is that as a result of the abrasion of the coupling plates, the viscous fluid is polluted to an increasing extent.